Assessment of the Validity of the Double Superhelix Model for Reconstituted High Density Lipoproteins

Abstract

High density lipoproteins (HDL) represent a heterogeneous population of particles with apoA-I as the major protein (1). Whether apoA-I/HDL 2 plays a direct role in cardiovascular disease prevention (e.g. removal of cholesterol from clogged arteries) or an indirect one (e.g. acts as a platform for the clustering of protective molecules, such as anti-inflammatory or antioxidant proteins), detailed knowledge of HDL structure is a key to understanding the molecular mechanism underlying these processes. Because the conformation of apoA-I on HDL is highly plastic (1), understanding apoA-I/HDL structure and dynamics is not straightforward.Since the early 1970s, the standard model for HDL particles reconstituted from apolipoprotein A-I (apoA-I) and phospholipid (apoA-I/HDL) has been that of a discoidal particle on the order of 100 Å in diameter and the thickness of a phospholipid bilayer. The initial observation of discoidal HDL particles was made by Forte et al. (2) when they examined HDL from patients with familial lecithin:cholesterol acyltransferase deficiency by negative stain EM. Reconstituted apoA-I/HDL particles (3) and nascent HDL from lymph (4) were then observed by negative stain EM to also have a discoidal shape. X-ray and neutron scattering studies (5, 6) provided further support for the standard discoidal model. The standard discoidal model has been used to interpret the results of a large number of experimental studies of the structure of reconstituted apoA-I/HDL particles. For review, see however, Wu et al. (11) used small angle neutron scattering to develop a model for apoA-I/HDL particles that is dramatically different from the standard model. In their model, termed a double superhelix (DSH), apoA-I possesses an open helical shape that twists around a central prolate ellipsoidal particle resembling a partial type I hexagonal lyotropic liquid crystalline phase.The DSH model is interesting. It is similar to MD simulation-based models (12) in that the proposed double superhelix forms a left-handed spiral as it wraps around the prolate ellip-

The low-resolution structure of nHDL reconstituted with DMPC with and without cholesterol reveals a mechanism for particle expansion

“…Second, in all contrast variation SANS-based models, the protein conformation is contorted and out of plane, a feature that is likely imparted by the presence of repeating proline residues which accentuate the bends in conformation. Of note, some discoidal models obtained by simulation bend out of plane, but they always remain closed loops (16)(17)(18). Another structural feature that sets apart the contrast variation SANS (direct experimental observation) versus simulation (hypothetical) models is the conformation of the N/C-termini.…”

“…All current models have in common an antiparallel orientation of the two apoA1 chains, but the overall conformation of apoA1 within the particle is still a matter of intense debate ( 18 ). The amphipathic apoA1 chains in all models wrap around a central lipid core, and the orientation of amino acid residues along the apoA1 chains position the hydrophobic side chains toward the lipid surface.…”

Structure of HDL: Particle Subclasses and Molecular Components

“…However, unlike in the planar disc, the Nand C-termini are not predicted interact in the DSH. The viability of this model remains under debate as molecular dynamics studies suggest that this model rapidly collapses to a disc-shaped structure (Jones et al 2010). Even simulations done by the group that proposed the model seem to indicate that the elongated structure collapses to an oblate spheroid, though they argue it remains micellar (Gogonea et al 2010).…”

“…Even simulations done by the group that proposed the model seem to indicate that the elongated structure collapses to an oblate spheroid, though they argue it remains micellar (Gogonea et al 2010). Going forward, this model must be squared with observations of disclike shapes by electron microscopy (Zhang et al 2011), EPR data (Jones et al 2010), and the quantised changes in particle size that can be readily demonstrated by reconstituting different lipid to protein ratios (Wald et al 1990). …”

New insights into the determination of HDL structure by apolipoproteins

“…Given these uncertainties, the validity of models of discoidal HDL particle structure showing apoA-I organizations that differ signifi cantly from the double-belt arrangement ( Figs. 6C and 7 ) is questionable ( 94 ).…”